Early on the morning of June 30,
1908, a massive explosion shook
central Siberia. Witnesses told
of a fireball that streaked in from
the southeast and then detonated in the
sky above the desolate, forested region.
At the nearest trading post, about 70 kilometers away from the blast, people were
reportedly knocked from their feet. Seismic instruments in the area registered
ground motions equivalent to those of a
magnitude-5 earthquake.

Effects of the event — often called the
Tunguska blast, after a major river running through the area — weren’t restricted
to Siberia. Sensitive barometers in England detected an atmospheric shock wave
as it raced westward and then detected it
again after it traveled around the world.
High-altitude clouds that formed over the
region after the event were so lofty that
they caught light from beyond the horizon, illuminating the sky so much that
people at locales in Europe and Asia could
read newspapers outdoors at midnight.

A number of factors — including the
site’s remote location, World War I and
the Russian Revolution — prevented scientists from mounting an expedition to
the blast zone for almost two decades, says
physicist Giuseppe Longo of the Univer-

The Tunguska blast shook Siberia in 1908, but on-site investigations were delayed for two
decades. One of the first photos showed a large area of flattened trees.

sity of Bologna in Italy. When researchers
eventually reached the region, they found
that a 2,150-square-kilometer patch of
forest had been flattened, with most of
the 80 million trees lying in a radial pattern. What the researchers didn’t find,
however, was an obvious crater.

A century later, scientists are still
debating the cause of the Tunguska blast.
Through the years, Longo notes, a variety
of scenarios have been proposed, many
of them involving the explosion of an
unusual extraterrestrial object — everything from a small black hole or a chunk
of antimatter to a UFO. Most researchers,
however, now pin the blame on the midair explosion of a small comet or asteroid,
which typically can’t stand up to the pummeling received while blazing through the
atmosphere (SN: 7/19/03, p. 36).

The damage in Siberia suggests that
the Tunguska detonation happened at an
altitude of bet ween 6 and 8 kilometers and
released the energy of about 15 megatons
of TNT, about a thousand times more than
the bomb that destroyed Hiroshima.

Data gathered by military satellites — including those designed to detect
clandestine nuclear explosions — suggest
that tiny versions of the Tunguska blast
occur rather frequently, says Philip A.
Bland, a planetary scientist at Imperial College London. The largest airburst detected
during the 1990s measured only a few tens
of kilotons, the energy release expected
from the explosion of an asteroid measuring about 7 or 8 meters across. Impacts of
objects measuring at least 1 meter in diameter occur, on average, about once a week,
the data suggest. Tunguska-sized airbursts »